1. Field of the Invention
The invention relates generally to the field of measurement while drilling (MWD) systems. More particularly, the invention relates to devices for communicating electrical power and sensor signals to and from sensors mounted proximate an external wall of a drill collar.
2. Background Art
MWD systems known in the art are used to make measurements of various drilling parameters and earth formation characteristics during the drilling of a wellbore. These measurements include, for example, the trajectory of the wellbore (inferred from measurements of trajectory of the MWD system based on the earth's gravity and its magnetic field), shock and vibration magnitude (inferred from acceleration measurements and/or strain measurements), and torque and axial loading applied to the collar (inferred from strain on the drill collar along various directions).
To make such measurements, MWD systems include various types of sensors and transducers mounted proximate the exterior wall of a drill collar in which the MWD system is disposed. Signals from the sensors are communicated to a signal processing and telemetry unit forming part of the MWD system. The signal processing and telemetry unit operates a transmitter which sends signals to a receiver at the earth's surface. These signals are typically in the form of modulation of the flow of drilling fluid (drilling mud) used to drill the wellbore. The signals represent the measurements made by the various sensors. Some of the measurements may also be stored in a recording device or memory in the signal processing and telemetry unit for later recovery when the MWD system is removed from the wellbore.
Some types of MWD systems are mounted in a mandrel, or similar housing, which is adapted to be removed from the interior of the drill collar for repair and maintenance. Using a mandrel type housing for the MWD system with sensors mounted near the exterior wall of the drill collar requires various types of electrical feed through devices to conduct signals from the sensors to appropriate circuits in the MWD mandrel. These electrical feed through devices also conduct electrical power to the sensors when such is needed. Electrical feed through devices can make repair and maintenance of the MWD system difficult and expensive. What is needed is a device which can eliminate the need to use electrical feed through devices in an MWD system.